Cored wire for the metallurgical treatment of a bath of molten metal and corresponding method

ABSTRACT

A cored wire ( 1; 100 ) intended to be injected into a molten metal bath to perform metallurgical treatment, the cored wire comprising: a fill ( 2 ) extending locally along a longitudinal axis (L), the fill comprising at least one active substance to treat the molten metal; and an outer sheath ( 4 ) extending longitudinally around the fill, characterized in that the fill comprises: an extruded bar ( 8 ) extending longitudinally and comprising the active substance; and an intermediate layer ( 10 ) extending longitudinally between the extruded bar and the outer sheath and comprising a powder containing one or more from among: a metal, a mixture of metals, a metal oxide, a mixture of metal oxides.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a cored wire intended to be fed into a bath ofmolten metal to perform metallurgical treatment, the cored wirecomprising:

-   -   a fill extending locally along a longitudinal axis, the core        comprising at least one active substrate to treat the molten        metal; and    -   an outer sheath extending longitudinally around the fill.

The molten metal is iron or steel for example. The objective ofmetallurgical treatment may be to add at least one substance for exampleto the molten metal, the substance being intended to regulate thecomposition of the molten metal and/or the composition of theprecipitates or non-metallic inclusions it contains.

2. Description of Related Art

In metallurgy it is known to provide a said substance by means of “coredwires” wound in coils. The cored wire is generally composed of a fillcomprising the active substance in powder form packed in a metal sheathformed of metal having a composition compatible with that of the moltenmetal to be treated. When treating molten steel, this sheath is itselfadvantageously in steel.

The cored wire is fed into the bath of molten metal using an injectiondevice, generally automatic, which injects a precise length of coredwire at suitable speed.

For example, in the foundry sector to produce spheroidal graphite castiron it is known to use a cored wire to conduct nodularisation. Thisconcerns the adding of magnesium to change the shape of the graphiteparticles of the iron, from lamellar to spheroidal. The added substanceis generally a magnesium ferrosilicon alloy in powder form.

According to another example, in the steel-making industry, it is knownto treat steels with calcium. For example this treatment is intended tomodify the chemical composition of endogenous inclusions of alumina typeto obtain inclusions of calcium aluminate type. These inclusions,dispersed in the molten steel, are liquid at casting temperature. Theytherefore cannot adhere to the nozzle walls of a ladle or tundish of acontinuous casting installation. Castability is thereby improved as isthe final quality of the steel produced.

Numerous types of cored wire exist having a fill formed of pure calciumor calcium alloy powder, or a mixture of calcium and iron, evenaluminium powders. For example, the alloy commonly called CaSi (calciumsilicide) or the mixture of calcium and iron powders (generally calledCaFe) are widely used to fill cores.

Although the adding of a cored wire to a bath of molten metal is aclever way of adding the active substance to the molten metal, theefficacy thereof is sometimes limited. For cored wires containing CaFepowders used in steel-making for example the yield of calcium addition,defined as the amount of calcium found in the steel after injection ofthe cored wire divided by the amount of calcium injected by the consumedcored wire, is generally in the order of 10% to 15%, sometimes muchless. The low efficacy of calcium is essentially due to its lowvaporisation temperature. In the region of 1480° C., this vaporisationis generally lower than the working temperature of the liquid steelwhich means that the calcium vaporises when being added to the liquidsteel.

It is one objective of the invention to provide a cored wire achievingmore efficient metallurgical treatment whilst remaining competitivelypriced.

BRIEF SUMMARY OF THE INVENTION

For this purpose the subject of the invention is a cored wire of thetype described above wherein the fill comprises:

-   -   an extruded bar extending longitudinally and containing the        active substance; and    -   an intermediate layer extending longitudinally between the        extruded bar and the outer sheath and comprising a powder        containing one or more from among: a metal, a mixture of metals,        a metal oxide or mixture of metal oxides.

According to particular embodiments, the device may comprise one or moreof the following characteristics taken alone or in any possibletechnical combination:

-   -   the fill may further comprise a thermally insulating layer        extending longitudinally between the extruded bar and the        intermediate layer;    -   the extruded bar has an equivalent outer diameter D1 in a        transverse plane substantially perpendicular to the longitudinal        axis, the intermediate layer having an equivalent outer diameter        D2 in the transverse plane with D2 between 1.3 times and 6.2        times D1;    -   the outer sheath comprises a strip in steel, aluminium, copper,        nickel, or zinc, or in an alloy of two or more of these        elements;    -   the extruded bar mostly contains magnesium;    -   the powder of the intermediate layer mostly contains an alloy of        iron and silicon also comprising calcium and/or barium and/or        one or more rare earths;    -   the extruded bar mostly contains calcium;    -   the powder of the intermediate layer comprises a powder of iron,        or a mixture of iron powder and aluminium powder and/or        magnesium powder and/or slag powder;    -   the outer sheath is in metal;    -   the outer sheath mostly contains one or more elements from        among: steel, copper, aluminium, nickel, zinc;    -   the outer sheath has a cross thickness of between 0.2 and 0.7        mm;    -   the extruded bar has an equivalent outer diameter of between 2        and 10 mm;    -   the cored wire has an equivalent diameter of between 6 and 21        mm;    -   the extruded bar is substantially cylindrical with circular        base;    -   the intermediate layer is of general hollow cylinder shape, the        cylinder having a circular base;    -   the outer sheath is of substantially general tubular shape with        circular base.

By “equivalent diameter” of an element is meant the diameter of a dischaving a surface equal to the surface presented by the element in crosssection. If the given element has a circular cross section theequivalent diameter is equal to the ordinary diameter.

When the notion of equivalent diameter is used for an element it isimplied that the element is locally substantially cylindrical but doesnot necessarily have a circular base.

By “metallurgical treatment” is meant for example:

-   -   modification of the chemical composition of the molten metal;        and/or    -   modification of the properties of the metal obtained after        solidification of the molten metal due for example to        modification of the composition of the inclusions or        precipitates existing before treatment, or to the creation of        such inclusions or precipitates subsequent to treatment; and/or    -   modification of the population of inclusions contained in the        liquid metal with a view to improving the production thereof        (improved castability for continuous casting).

The invention further concerns a method for metallurgical treatment of amolten metal bath, the method comprising the step of inserting a coredwire such as described above in the molten metal bath.

According to particular embodiments, the method may comprise one or moreof the following characteristics taken alone or in any possibletechnical combination:

-   -   the molten metal is iron melt and the cored wire added is such        as described above;    -   the molten metal is steel melt and the added cored wire is such        as described above.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will be better understood on reading the followingdescription given solely as an example with reference to the appendeddrawings in which:

FIG. 1 schematically illustrates a cored wire of the invention inperspective;

FIG. 2 is schematic cross-section illustrating the cored wire shown inFIG. 1; and

FIG. 3 is a graph illustrating, for a particular application, the slowedfading of the active substance (magnesium) when the cored wire shown inFIGS. 1 and 2 is injected in a molten metal bath (iron melt).

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIGS. 1 and 2, a cored wire 1 is described extendinglocally along a longitudinal axis. Solely one portion of the cored wire1 is shown. The illustrated portion extends along the longitudinal axisL. This does not mean that the entire cored wire 1 extends along thelongitudinal axis L The cored wire 1 may have a certain curve, forexample if it is coiled to take up less space.

Similarly, a transverse plane T is defined perpendicular to thelongitudinal axis L. It is understood that the transverse plane T istransverse for the illustrated portion of cored wire 1, i.e. locallytransverse.

The cored wire 1 is intended for example to be fed into a bath of molteniron (not illustrated).

The cored wire 1 comprises a fill 2 and an outer sheath 4 both extendinglongitudinally.

The outer sheath 4 forms a peripheral portion of the cored wire 1,intended to be in contact with the molten metal bath when the cored wire1 is inserted in the bath of molten metal.

The outer sheath 4 is advantageously formed of metal strip 6 folded overaround the longitudinal axis L.

The strip 6 is in steel, copper, aluminium, nickel or zinc for example,or else a mixture of two or more of these elements.

The strip 6 advantageously comprises two longitudinal folds 6 a, 6 b(FIG. 2) stapled together so that the strip 6 is closed along thelongitudinal axis L. Thus folded over the strip 6 is of general tubularshape surrounding the fill 2. Advantageously the tubular shape issubstantially cylindrical with a circular base and has an equivalentdiameter D. Advantageously D is between 6 and 21 mm. For example D is 13mm.

The fill 2 comprises an extruded bar 8 extending longitudinally and anintermediate layer 10 extending longitudinally between the extruded bar8 and the outer sheath 4.

The extruded bar 8 is advantageously substantially cylindrical withcircular base. The extruded bar 8 has a diameter D1 in the transverseplane T, with D1 advantageously between 2 and 10 mm, e.g. 8 mm.

The extruded bar 8 comprises an active substance to treat the molteniron. The active substance is magnesium for example. Advantageously theextruded bar 8 mostly contains magnesium.

By “mostly” is meant that the extruded bar 8 comprises at least 50% byweight of magnesium, preferably at least 90% by weight of magnesium.

In the example, the extruded bar 8 is formed of magnesium havingindustrial purity e.g. 99.8 weight %.

The extruded bar subsequent to treatment 8 is not a mere cluster ofpowder material compacted at the time the cored wire 1 is closed, nor anagglomerate of powder grains (powder material) bound together by abinder irrespective of kind. The extruded bar 8 is obtained for exampleby extrusion of a solid cylinder of material (billet) through a die bymeans of a press. The bar 8 can also be obtained directly using acontinuous casting method, the liquid material being solidified in theform of a continuous bar. The porosity of the extruded bar 8 isconsidered to be practically zero, the apparent density of the bar beingclose to the true density of the material.

The intermediate layer 10 extends for example in the space lying betweenthe extruded bar 8 and the outer sheath 4.

The intermediate layer 10 has an equivalent outer diameter D2. Forexample D2 is such that the ratio D2/D1 is between 1.3 and 6.2.

The intermediate layer 10 is advantageously formed of a powder. Theintermediate layer 10 may also comprise a thermally insulating layercovering the bar 8.

The intermediate layer 10 also mostly contains, as defined above, anactive substance for metallurgical treatment, e.g. a ferrosilicon alloy.Advantageously the intermediate layer 10 may also comprise up to 12% byweight of calcium, barium and rare earths (lanthanum, cerium).

The composition of the powder of the intermediate layer 10 is evidentlydependent on the metallurgical treatment to be conducted. It may beneutral i.e. having no metallurgical effect on the bath of liquid metalto be treated, in this case the powder solely acts as thermal insulatorof the bar 8. It may also take direct part in metallurgical treatmentand thereby assume a dual role of thermal insulator and active treatmentelement.

A description is now given of the functioning and use of the cored wire1 to carry out nodularisation and inoculation of the iron melt to obtainspheroidal graphite cast iron.

The iron to be treated is in the form of a molten metal bath containedfor example in a vessel such as a ladle.

The cored wire 1 is fed into the iron melt bath under conditions knownper se.

The adding of magnesium to the iron melt leads to a series of physicaland chemical conversions, some of which are simultaneous and otherssuccessive:

-   -   melting of the magnesium which starts at 657° C. (for pure Mg),    -   boiling of the magnesium at 1053° C. (for pure Mg).

The desulfurizing reaction is the following:

Mg+FeS->Fe+MgS.

Simultaneously there occurs intense mixing of the iron melt by gaseousMg and deoxidation of the iron melt. Magnesium is an energeticdeoxidizer. Once desulfurizing and deoxidation are completed, theremaining magnesium is incorporated in the iron melt. Evidentlythroughout the entire duration of the process, some of the magnesiumvapours formed escape from the surface of the molten iron and are fullylost by oxidation in the slag or atmosphere, giving rise to theformation of magnesium oxide for example.

The yield of magnesium addition is defined as the ratio between firstthe difference in Mg content effectively found in the melt afterinjecting the cored wire 1 and the Mg content in the melt before addingthe cored wire 1, and second the theoretical amount of Mg added to themelt by means of the cored wire 1 if 100% of the added Mg shouldeffectively be found in the melt.

The magnesium contained in the extruded bar 8 plays a nodularising rolei.e. it allows particles of spheroidal graphite to be obtained in thecast iron.

By means of the above-described characteristics the cored wire 1achieves more efficient metallurgical treatment, in this examplenodularisation of the cast iron, whilst remaining competitively priced.

The intermediate layer 10 acts as heat protection for the extruded bar8, slowing down the rise in temperature of the magnesium contained inthe extruded bar 8. The positioning of the intermediate layer 10 aroundthe extruded bar 8 provides protection thereto. At the time of injectionof the cored wire 1 into the molten iron bath, the rise in temperatureof the magnesium is delayed through slowed heat transfer. The cored wire1 can therefore be inserted more deeply into the column of liquid iron.The contact time of the magnesium gas with the liquid iron is therebylengthened and therefore improves the yield of magnesium addition.

In addition, in the extruded bar 8, the magnesium has a reduced specificsurface area for heat transfer compared with the surface of a fillcomposed of mere powder grains. The specific surface area is no longerthe surface of the grains but the side surface of the extruded bar 8.This slows down the vaporisation of the magnesium, thereby improving theaddition yield and moderating the reaction of Mg with the iron melt.

In the example, the intermediate layer 10 also plays a metallurgicalrole. The intermediate layer 10 comprises a second active substance formetallurgical treatment, namely a ferrosilicon alloy for example. Thissecond active substance acts as inoculant. As is known inoculationregenerates graphite seeding potential after treatment with magnesium.This allows prevention of the formation of cementite and contributestowards obtaining the desired content of spheroidal graphite. Forferrite iron grades it also promotes the formation of ferrite byincreasing the density of spherules.

In addition, since the substance that is active for nodularisation i.e.magnesium is contained in the extruded bar 8, it is possible to add theactive substance in more regular fashion than with prior art cored wirescontaining powder magnesium for which the density and compaction aredifficult to control. The small variation in magnesium weight per metrein the cored wire 1 advantageously in the order of +/−2%, allows areduction in the dispersion of residual magnesium in the treated ironmelt and hence lowering of the consumption of cored wire for one samequantity of Mg effectively added to the melt, and an improvement in thequality of the parts cast from the treated iron melt in particular via areduction in porosities and/or oxide films. The weight per metre ofmagnesium in the cored wire of the invention is much more precise thanthe weight per metre of magnesium contained in standard cored wires i.e.produced from magnesium powder.

Finally, with a view to imparting maximum metallurgical treatmentefficacy to the cored wire 1, the ratio of the diameters D2/D1 isbetween 1.3 and 6.2. This range was determined using the followingcriteria.

For the insulating intermediate layer to be sufficiently efficient, itmust be sufficiently thick. The space between the extruded bar and theouter sheath must therefore be sufficiently wide to contain the powder.A D2/D1 ratio of 1.3 or higher guarantees the minimum space required forsufficient thermal protection of the powder in the intermediate layer.

A D2/D1 ratio of 6.2 or lower is based on both metallurgical andeconomic considerations. It allows a guaranteed minimum proportion ofactive substance (extruded bar 8) in relation to the insulatingsubstance. If the imbalance is too great this will generate major heatlosses in the molten metal bath to be treated (too much powder addedhaving regard to the added active substance), but also an increase inthe cost price of the cored wire.

Example 1

Samples P1 to P8 were prepared and tested. The results of the tests aregiven in Tables 1, 2a and 2b below. The objective of these tests was toprove the advantage of the cored wire 1 of the invention in terms ofyield of magnesium addition when treating cast iron melt.

Samples P1 to P4 were commercially available, conventional cored wiresfilled with powder material having the following characteristics:

-   -   cored wire containing powders of FeSiMg, FeSi and FeSiTR in a        mixture with the powder of pure magnesium;    -   weight per metre of the mixture in the wire: 212.2 g/m;    -   weight per metre of magnesium in the wire: 63.4 g/m i.e. 29.9%;    -   weight per metre of silicon in the wire: 82 g/m i.e. 38.6%;    -   weight per metre of rare earths in the wire: 7.6 g/m i.e. 3.6%;        and    -   thickness of steel strip: 0.39 mm.

Samples P5 to P8 were taken from a cored wire conforming to theinvention and having the following characteristics:

-   -   bar 8 in magnesium having 99.8% purity;    -   diameter D1=6.6 mm;    -   intermediate layer 10 formed of a mixture of FeSiBa, FeSiTR and        CaSi powders;    -   diameter D2: 12.8 mm;    -   weight per metre of the intermediate layer 10: 272 g/m;    -   weight per metre of magnesium: 58 g/m i.e. 17.6%;    -   weight per metre of silicon: 123.4 g/m i.e. 37.4%;    -   weight per metre of rare earths: 19.8 g/m i.e. 6%; and    -   thickness of steel strip: 0.40 mm.

All the samples were tested under the same conditions namely in ladleshaving a height to diameter ratio of 0.8. The injection rates wereadapted for each of the wires in order to maintain one same quantity ofmagnesium added per unit of time. For each sample, the yield ofmagnesium addition was calculated (Tables 2a and 2b).

TABLE 1 Injection conditions metal Temperature quantity weight aftertreat- length rate gpm Mg gpm Si of added added (kg) ment (° C.) (m)(m/min) (g/m) (g/m) Mg (Kg) Mg % P1 736 1463 16.3 20 63.5 82 1.04 0.141%Standard P2 771 1467 16.8 20 63.5 82 1.07 0.138% wire P3 747 1462 16.520 63.5 82 1.06 0.140% P4 806 1464 17.6 20 63.5 82 1.12 0.139% P5 8071459 17.8 22 58.0 115 1.03 0.128% Wire of P6 876 1451 19.1 22 58.0 1151.11 0.128% invention P7 864 1465 19.1 22 58.0 115 1.11 0.128% P8 8651462 18.9 22 58.0 115 1.10 0.127%

TABLE 2a Technical results of standard wire Theoretical Residual FurnaceFinal Actual Magnesium added Si % Mg % sufhur % S % added Si % yield P10.18% 0.029 0.014 0.014 0.18 20.6 Standard P2 0.18% 0.026 0.014 0.0130.17 20.2 wire P3 0.18% 0.030 0.014 0.012 0.20 21.4 P4 0.18% 0.021 0.0140.011 0.17 15.1 mean 19.3 range 6.2

TABLE 2b Technical results of the wire according to the inventionTheoretical Residual Furnace Final Actual Magnesium added Si % Mg %sulfur % S % added Si % yield P5 0.25% 0.033 0.014 0.013 0.23 25.8 Wireof P6 0.25% 0.034 0.014 0.012 0.23 26.9 invention P7 0.25% 0.037 0.0140.011 0.20 28.9 P8 0.25% 0.032 0.014 0.006 0.25 25.3 mean 26.7 range 3.6

The yield of magnesium addition obtained with samples P5 to P8 of theinvention was a mean of 26.7%, compared with 19.3% for samples P1 to P4,i.e. an improvement of +38%. In Tables 2a and 2b the “range” is thedifference between the highest addition yield and the lowest additionyield. For samples P5 to P8, the range was reduced by 42% compared withsamples P1 to P4. This improves the predictability of magnesiuminjection obviating with greater certainty the need for a secondinjection due to insufficient Mg addition, and hence allows lesserconsumption of active product and avoids extending production time. Thisshows that over and above the positive impact on magnesium yield at thetime of injection, the use of an extruded wire provides for betterregularity of magnesium weight per metre, the treatment is more regularand results for residual magnesium after treatment are less dispersed.This makes it possible to reduce the nominal magnesium content andtherefore, in addition to the economic advantage of reduced wireconsumption, the quality of the molten metal will be improved since itis known that a high level of magnesium which is nonetheless necessaryto obtain a GS structure, has negative side effects e.g. increasedshrinkage propensity.

The intermediate layer 10 may also play a major metallurgical role, forexample allowing limited the fading of magnesium over time i.e. adecrease in the magnesium content of the iron melt after injecting thecored wire. Since the solubility of magnesium in iron melt is limitedand since this solubility is a function of temperature, itsconcentration decreases continuously as a function of time.

Magnesium is also a powerful deoxidizer and desulfurizer. Mg has atendency to combine with oxygen to form inclusions of MgO, therebybecoming increasingly less efficient for nodularisation of graphitespherules. The effect of spheroidization decreases, at times until itbecomes insufficient. The graphite then changes over from a perfectlyspheroid shape to an irregular, ragged shape and finally vermicularshape if the magnesium content is too low. The term iron degeneration isthen used.

To oppose this fading of magnesium in the iron melt and to improvegraphite spheroidization, several technical solutions were applied. Forexample, the intermediate layer 10 may contain deoxidizing elementsother than magnesium, e.g. cerium, but also chemical elements fromGroups IIA and IIIA in the Periodic Table and/or inoculating elementssuch as silicon. The multiplication of graphite nodule seeding sitesallows many more spherules to be obtained and thereby limits thedegeneration essentially affecting the large graphite nodules.

Example 2

Table 3 lists the tested samples and reference is made to graph 3 givingthe results in which the curves C1 to C4 correspond to the change intime of the proportion of Mg remaining in the iron melt compared withthe Mg added via the cored wire:

-   -   reference: curve C1,    -   PFT25: curve C2,    -   PFT32: curve C3,    -   PFT40: curve C4.

Curve C5 corresponds to a lower limit below which it is not desired toexceed in order to guarantee optimal quality of the cast parts.

TABLE 3 Composition of the intermediate layer 10 Reference PFT40 PFT32PFT25 % Silicon 35 31 35 39 % Rare Earths 1.5 1.5 1.5 1.5 incl. % Cerium1.0 1.0 1.0 1.0 % Calcium 0 3.6 3.7 3.8 % Barium 0 4.4 5.3 6.0 Powderwt. per metre (g/m) 205 210 222 232 Strip thickness (mm) 0.39 0.39 0.390.39

The four types of cored wires were tested under the same operatingconditions 15 treatments per wire), namely:

-   -   a cylindrical treatment ladle having a height-to-diameter ratio        of the metal column of 1.5;    -   the weight of treated metal was 2.5 tonnes;    -   the temperature of the iron melt was about 1470 to 1495° C.;    -   the melt composition 3.70% C; 2, 40% Si; 0.006-0.013% S.

It was found that the cored wires PFT25, PFT32 and PFT40 according toone preferred variant of the invention promote slower magnesium fadingthan the reference. The addition of 6% barium to the intermediate layertherefore allows lengthening of the lifetime of the treated melt(limited residual value of Mg guaranteeing the quality of the castparts) by 15 minutes (compared with the reference for which it is only20 minutes).

The intermediate layer 10 surrounding the extruded bar 8 allows asignificant reduction in magnesium fading over time. It was shown thatan intermediate layer 10 formed of a powder comprising a combinationbetween the elements cerium, calcium and barium allows a longermagnesium residence time to be obtained in the iron melt.

This also allows guaranteed homogeneous quality of all parts cast fromthe iron melt treated in this manner, from the first to the last. Thecasting process may last several tens of minutes and it is thereforeimportant that the magnesium content should be above the limit valuewhen the last part is cast from the same ladle of treated iron melt.

According to one variant not illustrated, the cored wire 1 may comprisean insulating layer extending longitudinally between the extruded bar 8and the intermediate layer 10.

The insulating layer may comprise paper for example, or moistened paper,metallized paper or metal. The insulating layer allows adjustment of theglobal heat transfer coefficient between the molten metal bath and theextruded bar 8. Advantageously the insulating delays the completemelting of the cored wire 1.

With reference to FIGS. 1 and 2 a description is now given of a coredwire 100 which is a variant of the above-described cored wire 1. Unlessotherwise indicated, the cored wire 100 is similar to cored wire 1 whichmeans that the same FIGS. 1 and 2 are used to illustrate the cored wire1 and cored wire 100.

The cored wire 100 mostly differs through it chemical composition anduse.

The cored wire 100 is intended for example to be injected into a bath ofmolten steel (not illustrated).

For the cored wire 100, the outer sheath 4 is in steel. As a variant, itmay be in aluminium, nickel, zinc or copper.

The extruded bar 8 mostly contains calcium. Preferably the extruded bar8 is formed of calcium having industrial purity of 98.5%. According toone variant (not illustrated) the extruded bar 8 can be encased in athermally insulating layer extending longitudinally.

The intermediate layer 10 comprises iron powder. As a variant it maycomprise powders of aluminium, magnesium and/or oxides such as slag.

For example:

the outer sheath 4 has a thickness of about 0.4 mm;

the weight per metre of the iron powder is about 300 g/m;

the weight per metre of the extruded bar 8 is about 85 g/m and has adiameter of about 8.5 mm.

The cored wire 100 is used in similar manner to the cored wire 1 e.g. totreat a molten steel bath with calcium.

One advantage of the cored wire 100 is that it develops the same weightper metre of calcium as a standard 30% CaFe cored wire (mixture ofcalcium and iron powders in proportions of: 30% Ca-70% Fe). It cantherefore be used as a direct replacement of standard CaFe cored wires,with an increased performance level in terms of yield of calciumtreatment in the ladle of liquid steel and reduced standard deviation ofyield i.e. improved predictability.

Example 3

Standard calcium treatment of a ladle of molten steel was conducted byinjecting a prior art CaFe cored wire A ladle of 245 tonnes was used.The targeted calcium content of the steel before being sent forcontinuous casting was 27 ppm.

The outer sheath of the cored wire had a thickness of 0.4 mm. The fillof the cored wire was a mixture of calcium and iron powders in a weightproportion of 30:70. The weight per metre of the powder mixture was 275g/m.

The mean length of the injected cored wire was 620 m, at an injectionrate of 290 m/min.

The mean addition yield was 12.9%. The standard deviation obtained inthe tests was 7.6% (absolute percentage).

152 ladles of the same molten steel were then treated with the coredwire 100 of the invention.

The sheath 4 of the cored wire 100 had a thickness of 0.4 mm. The fillof the cored wire 100 was a bar 8 of calcium having a diameter D1 of 8.5mm and weight per metre of 85 g/m and an iron powder surrounding thisbar 8 having a weight per metre of 300 g/m.

The diameter D of the cored wire 100 was similar to the diameter of thestandard cored wire, namely 13.6 mm.

The mean length of the injected cored wire 100 was about 374 m, at thesame injection rate of 290 m/min.

A mean addition yield of 20.8% was obtained with a standard deviation of4.3% (absolute percentage).

The treatment time of the steel was reduced by means of the cored wire100. On average the treatment lasted less than 80 seconds with the coredwire 100, compared with 130 seconds for the prior art CaFe wire.

Much higher mean addition yields were obtained: from 12.9% to 20.8%,i.e. an improvement of about +60%, with a lower standard deviation of4.3%.

The reduction in the amount of consumed cored wire 100 represents majorsavings in the cost of metallurgical treatment.

Finally a reduction in churning of the liquid metal was observed in theladle at the time of injecting the cored wire 100. This reduced churningallows for easier treatment of the ladles, the ladles having a low guardheight (distance between the upper edge of the ladle and the surface ofliquid metal), without the risk of metal splashes. The cored wire 100also allows a reduction in the maintenance frequency of the ladle coversince less metal remains adhering to the walls subsequent to spraying ofliquid metal. Additionally it reduces the reuptake of hydrogen in theliquid steel and re-oxidation thereof during calcium treatment, againdue to the lesser churning of the liquid metal which reduces itsexposure to the surrounding atmosphere.

As is conventional, a cored wire contains a powder or mixture of powdersfor which the weight per metre and composition must be controlledthroughout production. A conventional method for obtaining a cored wireparticularly comprises the following steps:

-   -   proportioning of each of the powders in accordance with the        desired composition of the fill;    -   mixing the powders in a mixing device; and    -   depositing the mixture to fill a cored wire.

These three steps determine the quality of the obtained cored wire.

The proportioning step of each of the powders allows the finalproportion to be heeded for each of the chemical elements forming thefill. However depending on the type of powder, this proportioning can beeasily perturbed. For example, when using a moving belt for transfer itis possible that one of the powders may be added in excess via drop-overeffect. When the belt stops, the powder that has reached the end of thebelt may continue to flow due to its inertia. This is all the morepossible the greater the flowability of the powder.

The mixing step is the most complex. Most mixers on cored wireproduction lines are of “ploughshare” type. Blades secured to a centralrotating shaft mix the different powders that were proportionedupstream. However mixers of this type easily induce segregationphenomena of the powders they are meant to mix. Depending on thedensities of the powders considered in relation to one another, somepowders have a tendency to accumulate in dead spots of the mixer whichlocally modifies the composition of the mixture. In addition, demixingphenomena between powders may also occur.

Finally, the depositing of the mixture in the cored wire at times causesheterogeneities in the mixture. At the time of deposit of the mixture,segregation sometimes occurs due in particular to the different pathwaystaken by the powder particles or to the phenomenon of elutriation.

The use of an extruded bar of the invention reduces the risk of poorproportioning of powders and poor mixing. The weight per metre of theextruded bar is much better controlled. Therefore the weight per metreof the active substance is much better controlled. For example thisweight per metre is independent of variations in density of the powdersused.

In the present application, by “thermally insulating layer” is meant anadditional layer around the extruded bar. The additional layer allowsdelaying of heat transfer from outside the cored wire towards the corewhen the cored wire is fed into a bath of molten metal. The additionallayer is adapted to form an additional heat barrier between the outsidemedium of the cored wire (liquid metal) and the extruded bar. Thepropagation of heat is slowed due to the presence of the additionallayer. The rise in temperature of the extruded bar is therefore delayed.

The efficacy of the thermally insulating layer varies in particular inrelation to type. Examples of thermally insulating layers are given inapplication FR-A-2871477 by the Applicant.

The fact that the thermally insulating layer is advantageously locatedon the extruded bar, and for example surrounds it completely, furtherimproves the thermal protection of the extruded bar.

1. A cored wire configured to be injected into a molten metal bath toperform metallurgical treatment, the cored wire comprising: a fillextending locally along a longitudinal axis, the fill comprising atleast one active substance to treat the molten metal; and an outersheath extending longitudinally around the fill; wherein the fillcomprises: an extruded bar extending longitudinally and comprising theactive substance; and an intermediate layer extending longitudinallybetween the extruded bar and the outer sheath and comprising a powdercomprising one or more selected from the group consisting of: a metal, amixture of metals, a metal oxide and a mixture of metal oxides.
 2. Thecored wire according to claim 1, wherein the fill further comprises athermally insulating layer extending longitudinally between the extrudedbar and the intermediate layer.
 3. The cored wire according to claim 1,wherein the extruded bar has an equivalent outer diameter D1 in atransverse plane substantially perpendicular to the longitudinal axis,the intermediate layer having an equivalent outer diameter D2 in thetransverse plane, with D2 between 1.3 and 6.2 times D1.
 4. The coredwire according to claim 1, wherein the outer sheath comprises a stripcomprising steel, aluminium, copper, nickel or zinc, or an alloy of twoor more of these elements.
 5. The cored wire according to claim 1,wherein the extruded bar mostly contains magnesium.
 6. The cored wireaccording to claim 5, wherein the powder of the intermediate layermostly comprises an alloy of iron and silicon, and also comprisescalcium and/or barium and/or one or more rare earths.
 7. The cored wireaccording to claim 1, wherein the extruded bar mostly comprises calcium.8. The cored wire according to claim 7, wherein the powder of theintermediate layer comprises an iron powder or a mixture of iron powderand aluminium powder and/or magnesium powder and/or slag powder.
 9. Amethod for metallurgical treatment of a molten metal bath, the methodcomprising a step of feeding a cored wire according to claim 1 into themolten metal bath.
 10. The method according to claim 9, wherein themolten metal is iron melt and wherein the fed cored wire is as describedby claim
 5. 11. The method according to claim 9, wherein the moltenmetal is steel and wherein the fed cored wire is as described in claim7.